The invention relates to a gas turbine with means for maintaining the turbine rotor blades at a low temperature. It particularly relates to a gas turbine wherein a hot gas is directed over only an annular portion of the turbine blades and a cooling gas is directed over another separate annular portion of the blades.
The efficiency of the gas turbine is greatly affected by the gas temperature and the efficiencies of the air compressor and the turbine. Significant developments in the field of aerodynamics have greatly improved the efficiencies of the compressor and turbines such that now one of the more important keys to improving thermal efficiencies of gas turbines is by elevating the inlet gas temperatures. However, the inlet gas temperatures are limited to those which can be withstood by the turbine materials of construction without a significant loss of structural integrity. At the present time most materials utilized in commercial gas turbines are limited to a maximum temperature of about 1800.degree. F. (1256.degree. K.) and generally operate at a temperature of about 1400.degree. F. (1033.degree. K.).
To permit the use of higher temperatures considerable research has been devoted to developing alloys which will withstand higher temperatures. In addition various means for cooling the turbine rotor or disc and blades to control their temperature have been proposed. For example, in U.S. Pat. No. 3,318,573 there is described a rotor wherein guide blades direct a flow of cooling air radially outward over the surface of the turbine disc in an attempt to maintain the disc at a low temperature. A similar approach is shown in U.S. Pat. No. 3,535,873 which describes a device for cooling gas turbine rotating members. In the device disclosed therein cool air from the stationary part of the engine is blown over the surface of the turbine disc, the amount of air being dependent upon the degree of cooling required.
In U.S. Pat. No. 3,904,307 there is disclosed another gas generator turbine cooling scheme which comprises an arrangement for cooling the high and low pressure rotors of a gas turbine. Cooling air is supplied from a source downstream of the rotors through the low pressure rotor to the space between them and from this space through the blades, and also to the space upstream of the high pressure rotor for cooling the face thereof.
In those proposals in which relatively cool air is directed through passageways formed in the turbine blades, the cooling effect provided by this air lowers the temperature of the blade material to a level at which melting or burning will not occur and at which the blade material has sufficient strength. One of the more difficult problems encountered in cooling turbine rotors in this fashion, however, is internally ducting the cooling air from the compressor to the turbine, which is rotating at high speeds. In usual practice the cooling air bypasses the engine combustor and is then introduced into passageways formed in the turbine rotor. Generally, the entrances to these rotor passageways have a peripheral speed of several hundred feet to as high as 2000 feet per second. Thus, it is inherent in the delivery of cooling air to the turbine in this fashion that a relatively large amount of work must be done on the cooling air to introduce it into the rotor. This work results in an increase in the temperature of the cooling air as it enters the turbine. This, in turn, reduces the cooling effectiveness of a given mass of air for reducing the temperature of the blades and other components of the turbine which are to be cooled thereby.
In an effort to overcome these problems it is proposed in U.S. Pat. No. 3,703,808 to provide a blade tip cooling air expander and seal system wherein cooling air is introduced into a chamber located above the blade tips. The cooling air passes through a plurality of nozzles located over the rotating blades. These nozzles accelerate the cooling air in the direction of wheel rotation and discharge it into a chamber which is bounded by stationary case hardware, the cooling air nozzles, a rotation shroud on the blade tips, and sealing elements located at the upstream and downstream ends of the blade shroud. The cooling air then flows into the cooling circuit formed with each blade from the chamber via a passageway through the blade shroud.
The problem with these cooling techniques is that they not only require a great deal of cooling air but frequently result in offsetting the cycle efficiency which was to be increased by elevating the temperature of the gas at the inlet of the turbine. More particularly, a great deal of work is required to compress the required amount of cooling gas. Further, the compression of the cooling gas raises its temperature, thus reducing its cooling capacity. Another problem is that the physical size restrictions of cooling passageways located inside the turbine blades limits the quantity of cooling gas that can be passed therethrough. In addition, these techniques frequently require complication construction of the rotating parts. Thus, there clearly is need for a gas turbine which can operate for sustained periods utilizing hot gases at temperatures above 3000.degree. F. (1922.degree. K.) or higher, which does not require expensive high temperature alloys for the materials of construction, complicated internal passageways, and cooling air workloads that offset the cycle efficiency gained by the use of higher temperature inlet gases.
In contrast to the types of gas turbines hereinbefore described, it is reported (K. Leist and E. Knornschild, NACA TM 1294, "Exhaust Turbine and Jet Propulsion Systems," translation of a 1939 German report) that a different type of gas turbine was developed and tested in Germany. Specifically, the gas turbine had hot gas directed over only a portion or preselected number of the turbine blades. The remainder of the turbine blades and rotor were exposed to ambient air for cooling. No further work is known to have been done on this type of turbine and it is believed that development was discontinued as a result of poor cycle efficiencies.